refactor(crd): replace raw REST client with controller-runtime client

[ivankatliarchuk]

What does it do ?

• replaced rest.Interface with client.Client + cache.Cache, deleted NewCRDClientForAPIVersionKind, List, watch, UpdateStatus, as supported by runtime interface out of the box
• rewrote buildCRDSource to create cache-backed controller-runtime client scoped to DNSEndpoint
• Added Indexing - Pre-filter objects when indexed, O(1) lookup
• replaced fake.RESTClient + HTTP mocking with fake.NewClientBuilder()

Motivation

Controller-runtime client provides all required functionality. Rest approach is very custom

Problems with REST Client Approach

• Client-Side Filtering - Fetched everything, filtered in memory
• Manual REST Construction - String-based resource names, error-prone, hard to add features
• No Caching - Every sync made full API calls (every 1 minute)
• Complex Setup - 40+ lines just to create a REST client
• Manual Goroutine Management - Easy to leak resources

Controller-Runtime Approach

• Built-in Caching - Zero API calls during Endpoints(), ~99% load reduction
• Type-Safe Client - &apiv1alpha1.DNSEndpoint{} vs string "dnsendpoints"
• Server-Side Filtering - Label/namespace filters applied at cache level
• Indexing - Pre-filter objects when indexed, O(1) lookup
• Managed Lifecycle - Context-based shutdown, no goroutine leaks

Relates

1. CRD Source Status field is lacking  #2092
2. Defining a path to Beta for DNSEndpoint API #5243
3. Add Record Creation Status to Source Object #6030

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

┌──────────┬───────┐
│ Category │ Lines │
├──────────┼───────┤
│ Non-test │ +125  │
├──────────┼───────┤
│ Test     │ +408  │
├──────────┼───────┤
│ Ratio    │ 1:3.2│
└──────────┴───────┘

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[coveralls]

Pull Request Test Coverage Report for Build 21798993330

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 45 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.02%) to 79.142%

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Files with Coverage Reduction	New Missed Lines	%
informers/informers.go	3	92.31%
informers/indexers.go	6	88.24%
crd.go	14	48.31%
store.go	22	50.6%

Totals
Change from base Build 21559764040:	-0.02%
Covered Lines:	16001
Relevant Lines:	20218

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💛 - Coveralls